This invention relates generally to a communication system which transmits signals on the neutral and ground lines of an AC power distribution network. More particularly, the invention relates to a noise trap circuit which is connected between the hot and neutral lines in order to improve the operation of the communication system.
U.S. Pat. No. 3,810,096 to Kabat, et al. and pending patent application Ser. No. 228,608 filed on Jan. 26, 1981 in the name of the same inventors (which application is a continuation of application Ser. No. 000,992, filed by the same inventors on Jan. 5, 1979 and now abandoned) disclose communication systems which operate in hotels and other buildings to transmit room status and related information on the existing lines of the power distribution network of the building. The system disclosed in U.S. Pat. No. 3,810,096 provides to the central desk area information concerning the status, price and availability of the various rooms of the hotel or motel. The system disclosed in the pending application is somewhat more versatile in that it permits two-way communication on the AC power lines between the central processing equipment and transceiver units which are located in the hotel rooms. Thus, inquiry can be made by the central processing equipment as to the status of a particular room or other area in the building or building complex. The benefits resulting from the use of these systems include reduced cost, improved service and increased convenience. Either type of system can be employed advantageously in hospitals, nursing homes, apartment complexes and other types of buildings as well as in hotels and motels. The communication occurs on the neutral and ground lines of the existing power distribution network, so there is no need for special additional wiring in new buildings or rewiring of existing buildings.
Although both types of communication systems have functioned well for the most part, the noise which is often present on the hot and neutral lines can interfere with the transmission of signals on the neutral and ground lines. In hotels and motels, the noise arises primarily from the operation of television sets, fluorescent lights, and electric razors and other devices powered by small electric motors. The present invention is aimed at reducing this noise on the hot and neutral lines in order to enhance the operation of the communication system.
In accordance with the invention, noise is reduced to a minimal level by installing noise traps between the hot and neutral lines of the AC power distribution network. Preferably, each noise trap circuit is an L-C-R series network connected across the hot and neutral lines and designed to absorb the most possible energy in the band of interest, i.e., the transmission band on the neutral and ground lines. The communication band width is typically about 20 kilocycles. For maximum noise attenuation, the resistor should have approximately the same impedance as the line impedance with the offending noise sources active. The reactive elements of the noise trap circuit should pass the communication signal band width. Although it is necessary for the circuit to include a capacitor if it is to effectively suppress the noise, the inductor can be eliminated in many cases without adversely affecting the noise trap.
In a preferred form of the invention, each transceiver which communicates with the power line is equipped with a noise trap. Since there is normally a transceiver in each hotel or motel room, the noise traps are distributed relatively uniformly throughout the building complex. Consequently, there is a noise suppressor physically near each offending noise source, and the noise can be effectively controlled due to the improved signal to noise ratio that results. In many cases, additional noise traps at selected locations in the building complex can enhance the overall quiteness of the power distribution network.